This invention relates to a method for manufacturing turborotors and, more particularly, this invention relates to a method for manufacturing gas turbine rotor wheels, and the improved rotor wheel resulting therefrom.
The problem encountered in the manufacture of highly stressed gas turbine rotor wheels is posed by the requirement for high strength at extremely high temperatures (1200.degree. C. and over). It has been found that hot strength properties like these are exhibited by ceramic or quasi-ceramic materials only, and it has therefore been attempted to manufacture such gas turbine wheel from ceramic materials. However, it was found that one-piece, fully ceramic gas turbine wheels will be destroyed as a result of thermal cracking under the elevated-temperature alternations of such gas turbine wheels. The ideal solution, therefore, would be gas turbine wheels where the rotor disk is made of materials that are highly resistant to thermal shock, while the blades are made of highly heat-resistant, preferably ceramic materials. Such a combination has in the past been difficult if not impossible to implement because all efforts securely attach ceramic or quasi-ceramic blades to a disk have failed. The lack of ductility of said ceramic materials or of cast, highly heat-resistant alloys keep the bearing contours of disk and blade root from matching perfectly, and this in turn causes extremely high concentrated loads destroying the blades.
Therefore, in a broad aspect, the present invention has for an object to provide a method for manufacturing turborotors where a durable blade-to-disk connection is achieved at reasonable expense.
It is a particular object of the present invention to provide a method by which the blade-to-disk connection is made by sintering the blade roots in place in the rotor disk.
The method according to the present invention provides a great advantage in that sintering achieves homogeneous support of the blade roots in the disk, so that load concentrations are avoided and use can be made, therefore, of very brittle blade materials.
In a preferred embodiment of the present invention the blade-to-disk connection is made by sintering the entire rotor disk while using premanufactured blades. In this embodiment of the present invention the premanufactured blades are inserted in circular succession in a die corresponding to the contour of the rotor disk, and after adding the sintering material the disk is sintered or hot-pressed to enclose the blade roots.
In a further embodiment of the present invention a one-piece bladed wheel is cast in a galvanoplastically made mold and then placed in a die for sintering the disk.
The inventive concept naturally also embraces gas turbine wheels made of premanufactured rotor disks having a cavernous recess for each blade, in which recess the blade root is inserted together with sintering material for sintering.
In a preferred aspect of the present invention the blades are cast blades of highly heat resistant materials.
The method of the present invention affords a particular advantage when it is intended to join ceramic blades, especially silicon ceramic blades, to a rotor disk, for the reason that, as previously described, there have been practically no reliable ways of attaching such ceramic blades to rotor disks. Imbedding such ceramic blades in the sintering material will eliminate stress concentrations that in the operation of said turborotors carry special risk for ceramic materials.
According to a further feature of the present invention the blade roots are coated with a ductile material before they are sintered in place in the rotor disk. Said coating serves to balance the widely different thermal expansions between the blade and the sintering material, so that excessive compressive stresses at the interface of disk and sintering material are prevented. As a coating material use is preferably made of metallic powders, such as niobium applied on the roots by metal spraying or in the form of aqueous solutions using a binding agent. Zirconium oxide has likewise proved to be of value as a coating material.
Another approach to preventing excessive compressive stresses as a result of widely different thermal expansions between the blade material and the sintering material, according to the invention, is to cover the blade roots with metal felt before sintering. The metal felt will then be sintered in place together with the root to serve, as do said coating materials, as a compressible cushion.
Sintering is preferably effected at a temperature somewhat below the recrystallization point, and the time at temperature will be minutes to hours, depending on the temperature used.
In a preferred aspect of the present invention the sintering pressure ranges from approximately 5,000 to 10,000 N/cm.sup.2.
A further advantage benefiting the strength of the blade-to-disk connection has been found to be provided when the lower portion of the blade root widens in pear-shaped manner in at least one plane, and space permitting, a pear-shaped widening provided in both an axial and a radial plane will create an even greater advantage. In this manner, the blade root will be surrounded and clamped on all sides for particularly good support of the blade.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.